1. Field of the Invention
The present invention relates to a picture quality adjusting circuit for use in a video signal processing apparatus, and, more particularly, to a picture quality adjusting circuit which uses a finite impulse response (FIR) filter to realize both contour compensation and noise canceling with a single circuit.
2. Description of the Related Art
A video signal processing apparatus that processes video signals of an NTSC system has a contour compensator and a noise canceling circuit or the like incorporated therein, the former for emphasizing the contour of a video image and the latter for reducing a noise component included in a video signal.
The contour compensator controls the gain of a signal of a frequency around 1 to 2 MHz, most detectible by the vision of human beings, in order to emphasize the contour of a picture. An example of the contour compensator has first and second differentiation circuits for acquiring a secondary differential waveform from an input signal, and a delay circuit for delaying the input signal. The output of the second differential circuit is inverted by an inverter and the inverted signal is added with the delayed input signal, thereby providing a contour-compensated signal.
An example of the noise canceling circuit has a high-pass filter for extracting a high-frequency component from an input signal, a limiter for extracting a noise component by limiting the level of the output of the filter, and a delay circuit for delaying the input signal. The noise component from the limiter is inverted by an inverter, and the resultant signal is added with the delayed, input signal, thereby providing a signal with the noise component canceled out.
The aforementioned conventional contour compensator is constituted by an analog circuit. In effect, therefore, the secondary differential waveform acquired from the contour compensator, for example, may include a ringing and the accurate contour compensation cannot be thus performed. Since the contour compensation is performed only on a luminance (Y) signal in order to protect a color (C) signal, it is necessary to provide the contour compensator at the succeeding stage to an Y/C separator. This restricts the circuit design.
Further, the conventional picture quality adjusting circuit requires the contour compensator and noise canceling circuit be arranged separately, thus enlarging the overall circuit be configuration.